1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device using an improved seal pattern and a fabricating method thereof.
2. Description of the Background Art
In general, a liquid crystal display (LCD) device utilizes the optical anisotropy and birefringence properties of liquid crystal molecules to display images. The liquid crystal display (LCD) device usually has first and second substrates spaced apart from and opposing each other, and a liquid crystal layer interposed therebetween. The first and second substrates respectively have electrodes for forming an electric field between the electrodes. That is, if a voltage is applied to the electrodes of the liquid crystal display (LCD) device, an electric field is formed between the electrodes and the electric field changes the alignment of the liquid crystal molecules. The changed alignment of the liquid crystal molecules control the light transmittance through the liquid crystal layer, and thus images can be displayed by controlling the light transmittance through the liquid crystal layer.
FIG. 1 is an exploded perspective view of a liquid crystal display (LCD) device according to the background art. As shown in FIG. 1, a liquid crystal display (LCD) device 11 has an upper substrate 5 having a black matrix 7, a color filter layer 8 and a common electrode 18 on the color filter layer 8, and a lower substrate 22 having a thin film transistor “T” and a pixel electrode 17 connected to the thin film transistor (TFT) “T.” A liquid crystal layer 14 is interposed between the upper and lower substrates 5 and 22. The lower substrate 22 is referred to as an array substrate because array lines including a gate line 13 and a data line 15 are formed thereon. The gate line 13 and the data line cross each other, and the TFT “T” of a switching element is disposed in a matrix and connected to the gate line 13 and the data line 15. The gate line 13 and the data line 15 define a pixel region “P” by crossing each other, and the TFT “T” is formed near a crossing portion of the gate line 13 and the data line 15. The pixel electrode 17 is formed of a transparent conductive material in the pixel region “P.” The upper substrate 5 is referred to as a color filter substrate because the color filter layer 8 is formed thereon.
The upper and lower substrates 5 and 22 are attached with a seal pattern (not shown) through a liquid crystal cell process. The seal pattern keeps a cell gap of the LCD device 11 uniform and prevents leakage of liquid crystal material injected into a space between the upper and lower substrates 5 and 22. The seal pattern is formed using a screen-printing method or a dispensing method using a sealant. The sealant is made of a heat curable epoxy resin or an UV (ultra violet) curable epoxy resin. Even though the epoxy resin itself does not damage to the liquid crystal material, the epoxy includes an amine that may dissolve into the liquid crystal material. Accordingly, if the seal pattern is formed of a heat curable epoxy resin, a sufficient pre-baking step is necessary under a gradual increase of temperature after the sealant is printed.
FIG. 2 is a schematic plane view showing a seal pattern on an array substrate for a liquid crystal display device according to the background art. As shown in FIG. 2, a seal pattern 2 formed on an array substrate 22 may be divided into two portions: a main seal line 2a and an auxiliary seal line 2b. The main seal line 2a keeps the cell gap uniform and prevents leakage of the liquid crystal material. After the array substrate 22 and a color filter substrate (not shown) are attached, a cleaning step and an etching step for the attached substrates are performed. The auxiliary seal line 2b protects the main seal line 2a from the cleaning solution and the etching solution used during the cleaning and etching steps.
FIG. 3 is a schematic cross-sectional view, which is taken along a line III-III of FIG. 2, showing a thin film transistor and a seal pattern of a liquid crystal display device according to a first embodiment of the background art. For example, an inverted staggered-type switching element is used in FIG. 3.
In FIG. 3, a gate line 13 (of FIG. 1) and a gate electrode 32 protruding from the gate line 13 (of FIG. 1) are formed on a lower substrate 22. The gate line 13 (of FIG. 1) and the gate electrode 32 are formed of a metallic material such as aluminum (Al), chromium (Cr) or molybdenum (Mo). A gate insulating layer 33 as a first insulating layer is formed on the gate electrode 32. The gate insulating layer 33 is formed of an inorganic insulating material such as silicon nitride (SiNx) or silicon oxide (SiO2). An active layer 36 of a semiconductor material is formed on the gate insulating layer 33 over the gate electrode 32. The active layer 36 has an island shape. Source and drain electrodes 39 and 41 are formed on the active layer 36. Even though not shown in FIG. 3, a data line 15 (of FIG. 1) crossing the gate line 13 (of FIG. 1) is simultaneously formed with the source and drain electrodes 39 and 41. The source electrode 39 is connected to the data line 15 (of FIG. 1) and the drain electrode 41 is spaced apart from the source electrode 39. The gate electrode 32, the active layer 36, the source electrode 39 and the drain electrode 41 constitute a thin film transistor (TFT) “T.”
A passivation layer 35 of an organic insulating material such as benzocyclobutene (BCB) and/or acrylic resin is formed on the TFT “T.” The passivation layer 35 has a drain contact hole 35a exposing the drain electrode 41. A pixel electrode 38 is formed on the passivation layer 35 and connects to the drain electrode 41 through the drain contact hole 35a. 
The lower substrate 22 attaches to an upper substrate 5 with a seal pattern 2. The seal pattern 2 is disposed between a common electrode 18 of the upper substrate 5 and the passivation layer 35 of the lower substrate 22. Since the seal pattern 2 is formed from a heat curable epoxy resin, it has a poor adhesion to the passivation layer 35 formed from an organic material, and defects such as a breakdown of the seal pattern may occur. Moreover, a stain may form at a portion near the seal pattern 2 because the seal pattern 2 has low resistance to moisture or contaminants from outside the display. Further, since the area buffering stress is small, a thin film may peel or come off due to the stress. To improve the adhesion, a structure of the seal pattern according to another embodiment of the background art has been suggested.
FIG. 4 is a schematic cross-sectional view, which corresponds to a portion “F” of FIG. 3, showing a seal pattern of a liquid crystal display device according to a second embodiment of the background art. As shown in FIG. 4, a seal pattern 2 having a width “W” is formed on a passivation layer 35 over a lower substrate 22. To improve the adhesion of the seal pattern 2, the passivation layer 35 and a gate insulating layer 33 are formed to have a groove 37. As a result, the seal pattern 2 on the passivation layer 35 contacts the gate insulating layer 33 formed of an inorganic material through the groove 37. Since the contact area of the seal pattern 2 and the passivation layer 35 is reduced and the seal pattern 2 contacts the gate insulating layer 33, the adhesion is improved.
However, since the region for the seal pattern 2 is limited, considering the aperture ratio of the LCD device, sufficient adhesion is not obtained using the structure of FIG. 4. To increase the contact area in a limited region, a structure of the seal pattern according to another embodiment of the background art has been suggested.
FIG. 5 is a schematic cross-sectional view, which corresponds to a portion “F” of FIG. 3, showing a seal pattern of a liquid crystal display device according to a third embodiment of the background art. As shown in FIG. 5, a gate insulating layer 33 of an inorganic material and a passivation layer 35 of an organic material are sequentially formed on a lower substrate 22. A seal pattern 2 is formed on the passivation layer 35. Since the gate insulating layer 33 and the passivation layer 35 have multiple grooves 39, the seal pattern 2 contacts the gate insulating layer 33 through the multiple grooves 39. The adhesion of the seal pattern 2 to the gate insulating layer 33 is better than the adhesion to the passivation layer 35. Moreover, the seal pattern 2 contacts the gate insulating layer 33 not only at a bottom portion, but also at a side portion of each groove 39. Accordingly, as the number of the grooves 39 increases, the total contact area also increases. As a result, the total contact area of the seal pattern 2 in FIG. 5 is larger than that in FIG. 4.
However, since the seal pattern 2 also contacts the passivation layer 35, a liquid crystal layer may be contaminated due to a chemical reaction between the seal pattern 2 and the passivation layer 35 at a contact portion “C.” Furthermore, the chemical reaction of the seal pattern 2 and the passivation layer 35 may cause a stain at a portion near the seal pattern 2 according to the curing degree of the passivation layer 35 and the chemical resistance of the seal pattern 2 under high temperature or high moisture conditions.
FIG. 6 is a photograph image showing a white stain of a liquid crystal display device constructed according to the background art. As shown in FIG. 6, a white stain “A” has generated at a periphery of the LCD device. Contamination of the liquid crystal layer due to a chemical reaction between the passivation layer and the seal pattern probably caused the white stain “A.”
FIG. 7 is a schematic plane view showing a position of a white stain according to the background art. As shown in FIG. 7, multiple liquid crystal cells “L1” to “L4” are disposed in an original substrate and a white stain “A” occurs at an left lower portion of one of the first to third liquid crystal cells “L1” to “L3.”
As has been shown, the chemical reactivity of the epoxy or acrylic seals tends to cause staining that results in the production of low quality liquid crystal displays. As a result, a technology that cleanly and effectively seals liquid crystal cells would be a great boon to the display industry.